What wiki says: "The tinge is believed to be caused by Mie scattering which occurs when incoming light with shorter wavelengths is preferentially scattered by particles within the atmosphere creating a blue-greyish colour to any distant objects, including mountains and clouds. Volatile terpenoids emitted in large quantities by the abundant eucalyptus trees in the Blue Mountains may cause Mie scattering and thus the blue haze for which the mountains were named."